I am not a proper source for tattoo information, these are just a few things I have learned.
SUGAR Having sugary lollies can help with the pain. Please do not do this if you are diabetic.
MUSIC Often studios will play music to listen to, otherwise you could bring your headphones. Songs you know and can sing along to always help.
MEDITATION OR CHANTING If you can chant or meditate in your head while getting tattooed it can help. It may not work for everyone. Swearing under your breath may also help, but I found chanting seemed to work better.
LAUGHTER If you have a tattoo artist you get on with and can laugh and joke, laughter also helps with pain.
HAVE A SYMPATHETIC ARTIST If it is your first time or you have a low pain threshold, like I seem to, make sure your tattoo artist is someone sympathetic who gets it and not someone mean who will just tell you to stop whining. My artist is great.
DON'T TAKE TOO LONG BREAKS Five to ten minutes to stretch and go to the toilet are fine, if you take too long a break you may not want to start again and your skin may be hurting more.
CHECKING IT OUT If you do take a short break, sometimes taking a peek in the mirror to see how awesome it is can help keep you motivated. Beware if the sight of your own blood makes you sick, though!
SHORTER STRETCHES Also consider doing your tattoo in shorter sessions, space these at least 2 weeks apart to allow for healing. I had one session for the outline (about 4hrs) and three for the colour (about 2 hours, 2.5 hours and 3.5 hours I think). I don't think I could take much longer.
BREATHE Sometimes when it really hurts, you realise you are holding your breath. As hard as it is, try to take deep breaths and relax.
FOOD Always eat before, so you will feel less dizzy. Also you won't be longing for it to be over so you can have a snack. I find having some carbs afterwards helps for me too, but it may not work for everyone.
FOLLOW CARE INSTRUCTIONS Your tattoo cost enough, take care of it when it is healing!
DON'T FREAK OUT during healing. Sometimes you will get bruises, and peeling tattoos look really gross, but it is all OK! Just don't pick it!
And obviously go to somewhere reputable!